refactor: overhaul 2048 merge animation and swap sound effects

- Replace PIECE_CLICK with alternating MASTERMIND_GLITCH sounds
- Refactor merge animation: redraw all tiles before merge phase instead
  of skipping merged positions
- Hide spawn tile early to prevent flash before spawn animation
- Simplify merge destination setup by removing redundant alpha reset
- Extract runMergePhase callback for cleaner merge/no-merge flow

public/src/games/2048/2048Game.js

@@ -35,11 +35,12 @@ export default class Game2048 extends Phaser.Scene {
     this.tileGraphics  = [];
     this.tileTexts     = [];
 
-    this.busy       = false;
-    this.gameOver   = false;
-    this.wonAlready = false;
-    this.score      = 0;
-    this.best       = 0;
+    this.busy        = false;
+    this.gameOver    = false;
+    this.wonAlready  = false;
+    this.score       = 0;
+    this.best        = 0;
+    this._sfxToggle  = 0;
 
     this.scoreValueText = null;
     this.bestValueText  = null;
@@ -416,7 +417,8 @@ export default class Game2048 extends Phaser.Scene {
     if (!newState) return;
 
     this.busy = true;
-    playSound(this, SFX.PIECE_CLICK);
+    playSound(this, this._sfxToggle ? SFX.MASTERMIND_GLITCH_2 : SFX.MASTERMIND_GLITCH_1);
+    this._sfxToggle ^= 1;
 
     this._animateMove(this.state, newState, () => {
       this.state = newState;
@@ -500,7 +502,7 @@ export default class Game2048 extends Phaser.Scene {
         return;
       }
 
-      // Build map: merge destination index → moving source containers currently at that position
+      // Build map: merge destination → moving source containers sitting at that position
       const mergeSources = new Map();
       for (const mv of movingEntries) {
         if (!mergeSet.has(mv.toIdx)) continue;
@@ -509,33 +511,35 @@ export default class Game2048 extends Phaser.Scene {
         mergeSources.get(mv.toIdx).push({ cont: this.tileContainers[fr][fc], fromIdx: mv.fromIdx });
       }
 
-      // Immediately update all tiles that aren't part of a merge transition
-      const skipIndices = new Set(newState.mergedAt);
-      for (const srcs of mergeSources.values()) {
-        for (const { fromIdx } of srcs) skipIndices.add(fromIdx);
-      }
-      this._redrawTilesExcept(newState, skipIndices);
-
-      // Count total source containers across all merges
       let totalSrcs = 0;
       for (const srcs of mergeSources.values()) totalSrcs += srcs.length;
 
-      if (totalSrcs === 0) {
+      // Restore every tile to its canonical position/value, then hand off to
+      // _showMergedTiles which overrides scale=0 on the merge destinations so
+      // the grow animation runs from nothing.  Hiding the spawn tile here
+      // prevents it flashing visible before spawnPhase animates it in.
+      const runMergePhase = () => {
+        this._redrawAllTiles(newState);
+        if (newState.spawned !== null) {
+          const sr = Math.floor(newState.spawned / 4), sc = newState.spawned % 4;
+          this.tileContainers[sr][sc].setAlpha(0);
+        }
         this._showMergedTiles(newState, spawnPhase);
-        return;
-      }
+      };
 
-      // Scale down all source containers simultaneously (they're at the merge position)
+      if (totalSrcs === 0) { runMergePhase(); return; }
+
+      // Scale down all merge-source containers simultaneously (they're at the merge position)
       let scaledDown = 0;
       const onAllScaledDown = () => {
-        // Reset every source to its home cell (invisible) before growing merged tiles
+        // Reset source containers to their home cells before the full redraw
         for (const [, srcs] of mergeSources) {
           for (const { cont, fromIdx } of srcs) {
             const { cx, cy } = this._cellCenter(Math.floor(fromIdx / 4), fromIdx % 4);
             cont.setAlpha(0).setScale(1).setPosition(cx, cy);
           }
         }
-        this._showMergedTiles(newState, spawnPhase);
+        runMergePhase();
       };
 
       for (const [, srcs] of mergeSources) {
@@ -577,9 +581,10 @@ export default class Game2048 extends Phaser.Scene {
     for (const idx of newState.mergedAt) {
       const tr = Math.floor(idx / 4), tc = idx % 4;
       const destCont = this.tileContainers[tr][tc];
-      this._paintTile(tr, tc, newState.grid[idx]);
+      // _redrawAllTiles already painted this tile at alpha=1 scale=1;
+      // collapse scale to 0 so the grow tween starts from nothing.
       this._scorePopup(destCont.x, destCont.y - 40, newState.grid[idx]);
-      destCont.setAlpha(1).setScale(0);
+      destCont.setScale(0);
       this.tweens.add({
         targets: destCont, scaleX: 1.2, scaleY: 1.2,
         duration: 200, ease: 'Back.easeOut',